1. Field of the Invention
The present invention relates to a condenser microphone, and more particularly to an electric circuit which can activate characteristics of an impedance converter housed therein, and adjust the frequency characteristic thereof while reducing devices to be used.
2. Description of the Related Art
When wind blows, or vibration arises, remarkable noises appear in audio signals. These noises are referred to as wind noise, and vibration noise. Frequency components of these noises reside in a low frequency range, so that when the lower side of frequency response characteristics of a microphone is made to attenuate, it becomes possible to suppress the appearance of the above-described noises and to make easy the collection of target sounds.
In a microphone unit housing an impedance converter as in a condenser type microphone unit, frequency characteristics are decided generally dependent on an electric acoustic converter, i.e. a microphone capsule (hereinafter referred to as “mic capsule”) which is located on the upstream of the above-described impedance converter. Furthermore, there is known such a method that the level of signals output from a drain side of a FET constituting an impedance converter is adjusted in every frequency range by means of a CR filter composed of a capacitor and a resistor, whereby the signal level is attenuated.
FIG. 4 is an example of a general circuit diagram showing a conventional condenser microphone. In FIG. 4, a microphone unit 1 includes a mic capsule 2 and a FET 3 functioning as an impedance converter. As is well-known, the mic capsule 2 is composed of a diaphragm made of a thin film and a counter electrode which is opposed to the diaphragm with a predetermined gap and constitutes the capacitor together with the diaphragm. The microphone unit 1 is constituted by containing a diaphragm holder, a circuit board and the like in addition to the mic capsule 2 into a unit case or the like. One of output terminals of the mic capsule 2 is connected to a gate of the FET 3, while the other end is grounded. The drain of the FET 3 is connected to a positive electrode of a power source Vcc through a resistor Rd, and a source of the FET 3 is grounded. An output terminal 5 is drawn out from the drain of the FET 3.
The frequency characteristics of the conventional example shown in FIG. 4 are decided by means of the mic capsule 2 as mentioned above. To adjust the frequency characteristics of the condenser microphone unit, it is necessary to add a filter circuit composed of, for example, a capacitor, a resistor and the like to the output terminal 5 or a site which is integrated electrically therewith. Furthermore, for the sake of suppressing or eliminating the above-described wind noise or vibration noise which gets mixed in the audio signal converted electroacoustically, the characteristics of the above-described filter circuit should be set so as to attenuate the frequency in low frequency range.
FIG. 5 shows another example of a conventional condenser microphone. The constitution of a microphone unit in FIG. 5 is the same as that of the conventional example shown in FIG. 4 wherein the same reference numerals are given to the same components as those of the conventional condenser microphones shown in FIGS. 4 and 5. In FIG. 5, a drain of a FET 3 is connected to the positive electrode of a power source Vcc, while a source of the FET 3 is grounded through a resistor R1. The source of the FET 3 is also connected to a base of a transistor Q1 through a capacitor C1 and an opening/closing switch 4, and a resistor R2 is connected in parallel to the switch 4. The switch 4 is a member for switching frequency characteristics of the microphone, and the switch 4 functions to open and close a route between the capacitor C1 and the base of the transistor Q1. As illustrated in FIG. 5, a resistor R2 is made to be effective thereby to obtain flat frequency characteristics in the case that the route is in an opened condition. When switched to a closed condition, the resistor R2 is made to be ineffective (short-circuited), whereby such frequency characteristics that the low frequency range thereof is allowed to attenuate are achieved. It is arranged in such that a collector of the transistor Q1 is connected to the positive electrode of the power source Vcc, an emitter is grounded through a resistor R5, and an output terminal 5 is connected to the emitter to output a signal from the emitter. A resistor R3 is connected across a base of the transistor Q1 and the positive electrode of the power source Vcc, while a resistor R4 is connected across the base and the ground. Accordingly, it is adapted in such that the power source voltage Vcc is partially pressurized by means of the resistors R4 and R3 to apply the voltage to the base of the transistor Q1.
In a conventional example shown in FIG. 5, the transistor Q1 functions as a buffer amplifier for preventing the influence of the input impedance of a microphone amplifier and the like which is to be connected on the downstream of the circuit. Although it is possible to adjust frequency characteristics by incorporating a filter circuit with the use of a CR without using the buffer amplifier, the performance is somewhat influenced. In the example shown in FIG. 5, although the output terminal 5 is drawn out from the source of the FET 3, it is possible to draw out the output terminal from the drain, or it is also possible to cut or reduce a low frequency range. The resistor R2 connected in parallel to a switch 4 functions also so as not to produce noises due to opening and closing the switch 4 by keeping a voltage between opposed ends of the capacitor C1 constant.
According to the conventional examples shown in FIGS. 4 and 5, it is required to constitute an electric circuit by adding a circuit element to the outside of the condenser microphone unit 1 for eliminating wind noise or vibration noise.
Incidentally, there is adopted a so-called plug-in power system wherein the power source of a variety of instruments in which a microphone is used, for example, that of a video recorder and the like is utilized also as the power source for the condenser microphone. The system functions in such that when a condenser microphone is mounted on an instrument such as a video recorder, a circuit is connected in a manner wherein a power source is supplied from that of the above-described instrument into the condenser microphone.
However, it is impossible to obtain such constitution that frequency characteristics can be adjusted in the instrument adopting the plug-in power system according to the conventional condenser microphones shown in FIGS. 4 and 5.
A variety of techniques by which frequency response of acoustical signals is suitably set or made to be variable in acoustical instruments is proposed (for example, see Patent Document 1, Patent Document 2, Patent Document 3, Patent Document 4, and Patent Document 5).
However, the inventions described in these Patent Documents do not have the peculiar constitution in the condenser microphone according to the invention of the present application, but no prior art of the condenser microphone relevant to the same technical idea as that of the invention of this application was found.
[Patent Document 1] Japanese Patent Application Laid-open No. 5-327380
[Patent Document 2] Japanese Patent Application Laid-open No. 10-241105
[Patent Document 3] Japanese Patent Application Laid-open No. 2007-129619
[Patent Document 4] Japanese Patent Application Laid-open No. 2001-189988
[Patent Document 5] Japanese Patent Application Laid-open No. 6-204755